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<p><strong><mark>所谓零拷贝不是不拷贝，而是没有CPU拷贝。</mark></strong></p>
<p>在 Java 程序中，常用的零拷贝有<code>mmap(内存映射)</code>和<code>sendFile</code>。那么，他们在 OS 里，到底是怎么样的一个的设计?我们分析 mmap 和 sendFile 这两个零拷贝</p>
<p>Java 传统 IO 和 网络编程的一段代码</p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20201108091644.png" alt="image-20201108091641597"></p>
<h3 id="传统-IO-模型"><a href="#传统-IO-模型" class="headerlink" title="传统 IO 模型"></a>传统 IO 模型</h3><p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20201108092614.png" alt="image-20201108092125492"></p>
<p><strong>DMA: direct memory access 直接内存拷贝(不使用 CPU)</strong></p>
<p><strong>传统IO经过了4次拷贝，4次切换。</strong></p>
<h3 id="mmap优化"><a href="#mmap优化" class="headerlink" title="mmap优化"></a>mmap优化</h3><p><strong>mmap 通过内存映射，将文件映射到内核缓冲区，同时，用户空间可以共享内核空间的数据。这样，在进行网络传输时，就可以减少内核空间到用户空间的拷贝次数。</strong>如下图</p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20201108092535.png" alt="image-20201108092533325"></p>
<p><strong>mmap优化后3次拷贝，4次切换。</strong></p>
<h3 id="sendFile优化"><a href="#sendFile优化" class="headerlink" title="sendFile优化"></a>sendFile优化</h3><p>Linux 2.1 版本 提供了sendFile函数，其基本原理如下:<strong>数据根本不经过用户态，直接从内核缓冲区进入到Socket Buffer，同时，由于和用户态完全无关，就减少了一次上下文切换</strong></p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20201108092857.png" alt="image-20201108092854624"></p>
<p>此时的拷贝次数为3.切换次数为3。但是还是存在cpy拷贝。</p>
<p>Linux 在 2.4 版本中，做了一些修改，避免了从内核缓冲区拷贝到 <strong>Socket buffer</strong> 的操作，直接拷贝到协议栈，从而再一次减少了数据拷贝。具体如下图和小结:</p>
<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20201108093152.png" alt="image-20201108093150230"></p>
<p><strong>3次切换，2次拷贝。</strong></p>
<p>这里其实有一次cpu拷贝<code>kernel buffer -&gt; socket buffer</code>。但是，拷贝的信息很少，比如 lenght , offset , 消耗低，可以忽略。</p>
<p><strong>我们说零拷贝，是从操作系统的角度来说的。因为内核缓冲区之间，没有数据是重复的(只有 kernel buffer 有一份数据)。零拷贝不仅仅带来更少的数据复制，还能带来其他的性能优势，例如更少的上下文切换，更少的 CPU 缓存伪共享以及无 CPU 校验和计算。</strong></p>
<h3 id="mmap和sendFile的区别"><a href="#mmap和sendFile的区别" class="headerlink" title="mmap和sendFile的区别"></a>mmap和sendFile的区别</h3><ol>
<li>mmap 适合小数据量读写，sendFile 适合大文件传输。</li>
<li>mmap 需要 4 次上下文切换，3 次数据拷贝;sendFile 需要 3 次上下文切换，最少 2 次数据拷贝。</li>
<li>sendFile 可以利用 DMA 方式，减少 CPU 拷贝，mmap 则不能(必须从内核拷贝到 Socket 缓冲区)。</li>
</ol>
<h2 id="NIO零拷贝案例"><a href="#NIO零拷贝案例" class="headerlink" title="NIO零拷贝案例"></a>NIO零拷贝案例</h2><p>NIO通过<code>transferTo()</code> 方法和<code>transferFrom()</code>方法实现零拷贝</p>
<p>传统IO</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//java IO 的服务器</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">OldIOServer</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> Exception </span>&#123;</span><br><span class="line">        ServerSocket serverSocket = <span class="keyword">new</span> ServerSocket(<span class="number">7001</span>);</span><br><span class="line"></span><br><span class="line">        <span class="keyword">while</span> (<span class="keyword">true</span>) &#123;</span><br><span class="line">            Socket socket = serverSocket.accept();</span><br><span class="line">            DataInputStream dataInputStream = <span class="keyword">new</span> DataInputStream(socket.getInputStream());</span><br><span class="line"></span><br><span class="line">            <span class="keyword">try</span> &#123;</span><br><span class="line">                <span class="keyword">byte</span>[] byteArray = <span class="keyword">new</span> <span class="keyword">byte</span>[<span class="number">4096</span>];</span><br><span class="line"></span><br><span class="line">                <span class="keyword">while</span> (<span class="keyword">true</span>) &#123;</span><br><span class="line">                    <span class="keyword">int</span> readCount = dataInputStream.read(byteArray, <span class="number">0</span>, byteArray.length);</span><br><span class="line"></span><br><span class="line">                    <span class="keyword">if</span> (-<span class="number">1</span> == readCount) &#123;</span><br><span class="line">                        <span class="keyword">break</span>;</span><br><span class="line">                    &#125;</span><br><span class="line">                &#125;</span><br><span class="line">            &#125; <span class="keyword">catch</span> (Exception ex) &#123;</span><br><span class="line">                ex.printStackTrace();</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br></pre></td></tr></table></figure>

<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">OldIOClient</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> Exception </span>&#123;</span><br><span class="line">        Socket socket = <span class="keyword">new</span> Socket(<span class="string">&quot;localhost&quot;</span>, <span class="number">7001</span>);</span><br><span class="line"></span><br><span class="line">        String fileName = <span class="string">&quot;protoc-3.6.1-win32.zip&quot;</span>;</span><br><span class="line">        InputStream inputStream = <span class="keyword">new</span> FileInputStream(fileName);</span><br><span class="line"></span><br><span class="line">        DataOutputStream dataOutputStream = <span class="keyword">new</span> DataOutputStream(socket.getOutputStream());</span><br><span class="line"></span><br><span class="line">        <span class="keyword">byte</span>[] buffer = <span class="keyword">new</span> <span class="keyword">byte</span>[<span class="number">4096</span>];</span><br><span class="line">        <span class="keyword">long</span> readCount;</span><br><span class="line">        <span class="keyword">long</span> total = <span class="number">0</span>;</span><br><span class="line"></span><br><span class="line">        <span class="keyword">long</span> startTime = System.currentTimeMillis();</span><br><span class="line"></span><br><span class="line">        <span class="keyword">while</span> ((readCount = inputStream.read(buffer)) &gt;= <span class="number">0</span>) &#123;</span><br><span class="line">            total += readCount;</span><br><span class="line">            dataOutputStream.write(buffer);</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        System.out.println(<span class="string">&quot;发送总字节数： &quot;</span> + total + <span class="string">&quot;, 耗时： &quot;</span> + (System.currentTimeMillis() - startTime));</span><br><span class="line"></span><br><span class="line">        dataOutputStream.close();</span><br><span class="line">        socket.close();</span><br><span class="line">        inputStream.close();</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20201108094613.png" alt="image-20201108094611968"></p>
<p>NIO零拷贝</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//服务器</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">NewIOServer</span> </span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> Exception </span>&#123;</span><br><span class="line"></span><br><span class="line">        InetSocketAddress address = <span class="keyword">new</span> InetSocketAddress(<span class="number">7001</span>);</span><br><span class="line"></span><br><span class="line">        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();</span><br><span class="line"></span><br><span class="line">        ServerSocket serverSocket = serverSocketChannel.socket();</span><br><span class="line"></span><br><span class="line">        serverSocket.bind(address);</span><br><span class="line"></span><br><span class="line">        <span class="comment">//创建buffer</span></span><br><span class="line">        ByteBuffer byteBuffer = ByteBuffer.allocate(<span class="number">4096</span>);</span><br><span class="line"></span><br><span class="line">        <span class="keyword">while</span> (<span class="keyword">true</span>) &#123;</span><br><span class="line">            SocketChannel socketChannel = serverSocketChannel.accept();</span><br><span class="line"></span><br><span class="line">            <span class="keyword">int</span> readcount = <span class="number">0</span>;</span><br><span class="line">            <span class="keyword">while</span> (-<span class="number">1</span> != readcount) &#123;</span><br><span class="line">                <span class="keyword">try</span> &#123;</span><br><span class="line"></span><br><span class="line">                    readcount = socketChannel.read(byteBuffer);</span><br><span class="line"></span><br><span class="line">                &#125;<span class="keyword">catch</span> (Exception ex) &#123;</span><br><span class="line">                   <span class="comment">// ex.printStackTrace();</span></span><br><span class="line">                    <span class="keyword">break</span>;</span><br><span class="line">                &#125;</span><br><span class="line">                <span class="comment">//</span></span><br><span class="line">                byteBuffer.rewind(); <span class="comment">//倒带 position = 0 mark 作废</span></span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">NewIOClient</span> </span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> Exception </span>&#123;</span><br><span class="line"></span><br><span class="line">        SocketChannel socketChannel = SocketChannel.open();</span><br><span class="line">        socketChannel.connect(<span class="keyword">new</span> InetSocketAddress(<span class="string">&quot;localhost&quot;</span>, <span class="number">7001</span>));</span><br><span class="line">        String filename = <span class="string">&quot;protoc-3.6.1-win32.zip&quot;</span>;</span><br><span class="line"></span><br><span class="line">        <span class="comment">//得到一个文件channel</span></span><br><span class="line">        FileChannel fileChannel = <span class="keyword">new</span> FileInputStream(filename).getChannel();</span><br><span class="line"></span><br><span class="line">        <span class="comment">//准备发送</span></span><br><span class="line">        <span class="keyword">long</span> startTime = System.currentTimeMillis();</span><br><span class="line"></span><br><span class="line">        <span class="comment">//在linux下一个transferTo 方法就可以完成传输</span></span><br><span class="line">        <span class="comment">//在windows 下 一次调用 transferTo 只能发送8m , 就需要分段传输文件, 而且要主要</span></span><br><span class="line">        <span class="comment">//传输时的位置 =》 课后思考...</span></span><br><span class="line">        <span class="comment">//transferTo 底层使用到零拷贝</span></span><br><span class="line">        <span class="keyword">long</span> transferCount = fileChannel.transferTo(<span class="number">0</span>, fileChannel.size(), socketChannel);</span><br><span class="line"></span><br><span class="line">        System.out.println(<span class="string">&quot;发送的总的字节数 =&quot;</span> + transferCount + <span class="string">&quot; 耗时:&quot;</span> + (System.currentTimeMillis() - startTime));</span><br><span class="line"></span><br><span class="line">        <span class="comment">//关闭</span></span><br><span class="line">        fileChannel.close();</span><br><span class="line"></span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p><img src= "https://gitee.com/kylincw/images/raw/master/loading.gif" data-lazy-src="https://qiniu.codekylin.cn/img/20201108094720.png" alt="image-20201108094719217"></p>
<p>这里的文件比较小所以区别不大。但是还是比传统IO要快的。</p>
<h2 id="Java-AIO基本介绍"><a href="#Java-AIO基本介绍" class="headerlink" title="Java AIO基本介绍"></a>Java AIO基本介绍</h2><p>JDK 7 引入了 Asynchronous I/O，即 AIO。在进行 I/O 编程中，常用到两种模式:Reactor 和 Proactor。Java 的 NIO 就是 Reactor，当有事件触发时，服务器端得到通知，进行相应的处理</p>
<p>AIO 即 NIO2.0，叫做异步不阻塞的 IO。AIO 引入异步通道的概念，采用了 Proactor 模式，简化了程序编写， 有效的请求才启动线程，它的特点是先由操作系统完成后才通知服务端程序启动线程去处理，一般适用于连接 数较多且连接时间较长的应用。</p>
<p>目前AIO还没有广泛应用，Netty也是基于NIO,而不是AIO，因此我们就不详解AIO了，有兴趣的可以参考 &lt;&lt;Java 新一代网络编程模型 AIO 原理及 Linux 系统 AIO 介绍 &gt;&gt;</p>
<p><a target="_blank" rel="noopener" href="http://www.52im.net/thread-306-1-1.html">http://www.52im.net/thread-306-1-1.html</a></p>
<h2 id="BIO、NIO、AIO-对比表"><a href="#BIO、NIO、AIO-对比表" class="headerlink" title="BIO、NIO、AIO 对比表"></a>BIO、NIO、AIO 对比表</h2><table>
<thead>
<tr>
<th></th>
<th>BIO</th>
<th>NIO</th>
<th>AIO</th>
</tr>
</thead>
<tbody><tr>
<td>IO模型</td>
<td>同步阻塞</td>
<td>同步非阻塞（多路复用）</td>
<td>异步非阻塞</td>
</tr>
<tr>
<td>编程难度</td>
<td>简单</td>
<td>复杂</td>
<td>复杂</td>
</tr>
<tr>
<td>可靠性</td>
<td>差</td>
<td>好</td>
<td>好</td>
</tr>
<tr>
<td>吞吐量</td>
<td>低</td>
<td>高</td>
<td>高</td>
</tr>
</tbody></table>
<p>举例说明</p>
<ol>
<li>同步阻塞:到理发店理发，就一直等理发师，直到轮到自己理发。</li>
<li>同步非阻塞:到理发店理发，发现前面有其它人理发，给理发师说下，先干其他事情，一 会过来看是否轮到自己.</li>
<li>异步非阻塞:给理发师打电话，让理发师上门服务，自己千其它事情，理发师自己来家给你理发</li>
</ol>
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